This invention relates generally to vertical compensation circuits for television receivers (TVs) and specifically to vertical compensation circuits for digital television receivers.
The above mentioned copending application describes the difficulty encountered in digital television receivers with changes in vertical size of the screen display or raster with fluctuations in beam current in the cathode ray tube (CRT). In analog TVs, a large degree of self compensation is incorporated by using a vertical oscillator having its B+ operating voltage supplied from the horizontal deflection and high voltage system. In that arrangement the amplitude of the sawtooth drive waveform produced by the vertical oscillator changes in the same direction as changes in the high voltage or B+ supply of the TV. Should the CRT beam current increase (and result in a decrease in the high voltage supplied to the CRT), the electron beam becomes "easier" to deflect. Without a compensating change in the amplitude of the vertical drive waveform, the vertical size of the raster will increase. By linking the vertical oscillator to a B+ supply that is tied to the high voltage supply, the amplitude of the vertical drive waveform is diminished and tends to compensate for the lower high voltage.
In digital TVs, there is no vertical oscillator. On the contrary, the vertical drive signal is derived by integration of a pulse width modulated (PWM) signal having a substantially constant amplitude that is independent of variations in the B+ and high voltage supplies. Therefore, a change in high voltage (or B+) does not affect the amplitude of the vertical drive signal, which remains substantially constant. The result is that the size of the raster, in the vertical direction, changes inversely with changes in B+ voltage (and high voltage). This creates an objectionable viewing situation. In the above mentioned copending application a solution to the problem entails provision of compensation means, responsive to changes in the B+ supply voltage, for changing the amplitude of the PWM signal prior to development of the vertical drive signal. The present invention provides an alternative solution to this problem by operating directly on the vertical drive signal to change its amplitude with B+ changes. Thus, in the present invention an amplifier in the vertical deflection system has its gain varied as a function of the B+ potential, which is related to the TV high voltage. Specifically, in the preferred embodiment of the invention the vertical deflection system includes a feedback circuit, the impedance of which is changed as a function of an output from voltage sensing means that sense changes in the B+ supply.